Long post ahead.
No it won't just be Sora as far as I know although who the other units are is uncertain I imagine they don't want to announce too early, although I want them no matter how shite or amazing they are.
Fingers crossed you get 2 Siegs finishing the step ups. BTW his story event is absolutely hilarious.
So from what I've heard from the Fan Festa news, they'll probably do enhancements and awakenings for the previous units, Christine,(magic chainer locked into ice element) Kryla (imperiler and debuffer) and White Knight Noel (Tank) will probably get 7* upgrades and further enhancements they were all limited units but extremely good with amazing TMRs usually most of the limited units are really good.
Upon release until T.Terra got her 7* Christine was tied with her for top tier magic damage and her TMR is still top tier for magic users +50 mag, +30 spr, can counter magic attacks with 15% HP recover and LB increase.
Kryla is pretty much on par with Ignis in terms of imperilling and debuffing and has a fab TMR for magic tanks, increases SPR by 15%, resistance to all elements by 25% and LB increase.
WKN is still an amazing Tank huge physical and magic mitigation except because he's limited and he lacks AOE abilities he's not mentioned so much, but if you set him up right you can make him 100% evade and provoke. He also has the unusual ability in a tank of being a decent magic attacker, there's been trials in the past where people have used just him and one DPS units to take bosses down his TMR is amazing for both physical tanks and magic users +50 Mag and Def.
They have two new units coming out for the Xmas banners called Tiana she's a quadcast magic attacker and Felix is going to be chainer, finisher TDH user etc no idea what else they're planning for them.